Java装箱和拆箱的基本概念及使用

2017-01-12 10:05:48来源:作者:CSDN博客人点击

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要理解装箱和拆箱的概念,就要理解Java数据类型

装箱:把基本类型用它们相应的引用类型包装起来,使其具有对象的性质。int包装成Integer、float包装成Float

拆箱:和装箱相反,将引用类型的对象简化成值类型的数据

Integer a = 100; 这是自动装箱 (编译器调用的是static Integer valueOf(int i))

int b = new Integer(100);这是自动拆箱

看下面一段代码

m1 public class DataType { public static void main(String args[]) { DataType dt = new DataType(); dt.m11(); dt.m12(); } public void m11() { Integer a = new Integer( 100 ); Integer b = 100 ; System.out.println( "m11 result " + (a == b)); } public void m12() { Integer a = new Integer( 128 ); Integer b = 128 ; System.out.println( "m12 result " + (a == b)); } }

打印结果是什么?

m11 result false

m12 result false

“==”比较的是地址,而a和b两个对象的地址不同,即是两个对象,所以都是false

通过javap解析字节码,内容如下

public void m11(); Code: 0: new #44; //class java/lang/Integer 3: dup 4: bipush 100 6: invokespecial #46; //Method java/lang/Integer."< init >":(I)V 9: astore_1 10: bipush 100 12: invokestatic #49; //Method java/lang/Integer.valueOf:(I)Ljava/lang/In teger; 15: astore_2 16: getstatic #53; //Field java/lang/System.out:Ljava/io/PrintStream; 19: new #59; //class java/lang/StringBuilder 22: dup 23: ldc #61; //String m11 result 25: invokespecial #63; //Method java/lang/StringBuilder."< init >":(Ljava/la ng/String;)V 28: aload_1 29: aload_2 30: if_acmpne 37 33: iconst_1 34: goto 38 37: iconst_0 38: invokevirtual #66; //Method java/lang/StringBuilder.append:(Z)Ljava/la ng/StringBuilder; 41: invokevirtual #70; //Method java/lang/StringBuilder.toString:()Ljava/l ang/String; 44: invokevirtual #74; //Method java/io/PrintStream.println:(Ljava/lang/St ring;)V 47: return public void m12(); Code: 0: new #44; //class java/lang/Integer 3: dup 4: sipush 128 7: invokespecial #46; //Method java/lang/Integer."< init >":(I)V 10: astore_1 11: sipush 128 14: invokestatic #49; //Method java/lang/Integer.valueOf:(I)Ljava/lang/In teger; 17: astore_2 18: getstatic #53; //Field java/lang/System.out:Ljava/io/PrintStream; 21: new #59; //class java/lang/StringBuilder 24: dup 25: ldc #82; //String m12 result 27: invokespecial #63; //Method java/lang/StringBuilder."< init >":(Ljava/la ng/String;)V 30: aload_1 31: aload_2 32: if_acmpne 39 35: iconst_1 36: goto 40 39: iconst_0 40: invokevirtual #66; //Method java/lang/StringBuilder.append:(Z)Ljava/la ng/StringBuilder; 43: invokevirtual #70; //Method java/lang/StringBuilder.toString:()Ljava/l ang/String; 46: invokevirtual #74; //Method java/io/PrintStream.println:(Ljava/lang/St ring;)V 49: return </ init ></ init ></ init ></ init > m2 public class DataType { public static void main(String args[]) { DataType dt = new DataType(); dt.m21(); dt.m22(); } public void m21() { Integer a = new Integer( 100 ); Integer b = new Integer( 100 ); System.out.println( "m21 result " + (a == b)); } public void m22() { Integer a = new Integer( 128 ); Integer b = new Integer( 128 ); System.out.println( "m22 result " + (a == b)); } }

打印结果是

m21 result false

m22 result false

a和b仍是两个对象

javap解析内容

public void m21(); Code: 0: new #44; //class java/lang/Integer 3: dup 4: bipush 100 6: invokespecial #46; //Method java/lang/Integer."<init>":(I)V 9: astore_1 10: new #44; //class java/lang/Integer 13: dup 14: bipush 100 16: invokespecial #46; //Method java/lang/Integer."<init>":(I)V 19: astore_2 20: getstatic #53; //Field java/lang/System.out:Ljava/io/PrintStream; 23: new #59; //class java/lang/StringBuilder 26: dup 27: ldc #84; //String m21 result 29: invokespecial #63; //Method java/lang/StringBuilder."<init>":(Ljava/lang/String;)V 32: aload_1 33: aload_2 34: if_acmpne 41 37: iconst_1 38: goto 42 41: iconst_0 42: invokevirtual #66; //Method java/lang/StringBuilder.append:(Z)Ljava/lang/StringBuilder; 45: invokevirtual #70; //Method java/lang/StringBuilder.toString:()Ljava/lang/String; 48: invokevirtual #74; //Method java/io/PrintStream.println:(Ljava/lang/String;)V 51: returnpublic void m22(); Code: 0: new #44; //class java/lang/Integer 3: dup 4: sipush 128 7: invokespecial #46; //Method java/lang/Integer."<init>":(I)V 10: astore_1 11: new #44; //class java/lang/Integer 14: dup 15: sipush 128 18: invokespecial #46; //Method java/lang/Integer."<init>":(I)V 21: astore_2 22: getstatic #53; //Field java/lang/System.out:Ljava/io/PrintStream; 25: new #59; //class java/lang/StringBuilder 28: dup 29: ldc #86; //String m22 result 31: invokespecial #63; //Method java/lang/StringBuilder."<init>":(Ljava/lang/String;)V 34: aload_1 35: aload_2 36: if_acmpne 43 39: iconst_1 40: goto 44 43: iconst_0 44: invokevirtual #66; //Method java/lang/StringBuilder.append:(Z)Ljava/lang/StringBuilder; 47: invokevirtual #70; //Method java/lang/StringBuilder.toString:()Ljava/lang/String; 50: invokevirtual #74; //Method java/io/PrintStream.println:(Ljava/lang/String;)V 53: return m3 public class DataType { public static void main(String args[]) { DataType dt = new DataType(); dt.m31(); dt.m32(); } public void m31() { Integer a = 100 ; Integer b = 100 ; System.out.println( "m31 result " + (a == b)); } public void m32() { Integer a = 128 ; Integer b = 128 ; System.out.println( "m32 result " + (a == b)); } }

打印结果

m31 result true

m32 result false

为什么有第一个是true,第二个是false呢?观察javap解析的数据

javap解析内容

public void m31(); Code: 0: bipush 100 2: invokestatic #49; //Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer; 5: astore_1 6: bipush 100 8: invokestatic #49; //Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer; 11: astore_2 12: getstatic #53; //Field java/lang/System.out:Ljava/io/PrintStream; 15: new #59; //class java/lang/StringBuilder 18: dup 19: ldc #88; //String m31 result 21: invokespecial #63; //Method java/lang/StringBuilder."<init>":(Ljava/lang/String;)V 24: aload_1 25: aload_2 26: if_acmpne 33 29: iconst_1 30: goto 34 33: iconst_0 34: invokevirtual #66; //Method java/lang/StringBuilder.append:(Z)Ljava/lang/StringBuilder; 37: invokevirtual #70; //Method java/lang/StringBuilder.toString:()Ljava/lang/String; 40: invokevirtual #74; //Method java/io/PrintStream.println:(Ljava/lang/String;)V 43: returnpublic void m32(); Code: 0: sipush 128 3: invokestatic #49; //Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer; 6: astore_1 7: sipush 128 10: invokestatic #49; //Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer; 13: astore_2 14: getstatic #53; //Field java/lang/System.out:Ljava/io/PrintStream; 17: new #59; //class java/lang/StringBuilder 20: dup 21: ldc #90; //String m32 result 23: invokespecial #63; //Method java/lang/StringBuilder."<init>":(Ljava/lang/String;)V 26: aload_1 27: aload_2 28: if_acmpne 35 31: iconst_1 32: goto 36 35: iconst_0 36: invokevirtual #66; //Method java/lang/StringBuilder.append:(Z)Ljava/lang/StringBuilder; 39: invokevirtual #70; //Method java/lang/StringBuilder.toString:()Ljava/lang/String; 42: invokevirtual #74; //Method java/io/PrintStream.println:(Ljava/lang/String;)V 45: return m4 public class DataType { public static void main(String args[]) { DataType dt = new DataType(); dt.m41(); dt.m42(); } public void m41() { Integer a = Integer.valueOf( 100 ); Integer b = 100 ; System.out.println( "m41 result " + (a == b)); } public void m42() { Integer a = Integer.valueOf( 128 ); Integer b = 128 ; System.out.println( "m42 result " + (a == b)); } }

打印结果

m41 result true

m42 result false

javap解析内容

public void m41(); Code: 0: bipush 100 2: invokestatic #49; //Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer; 5: astore_1 6: bipush 100 8: invokestatic #49; //Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer; 11: astore_2 12: getstatic #53; //Field java/lang/System.out:Ljava/io/PrintStream; 15: new #59; //class java/lang/StringBuilder 18: dup 19: ldc #92; //String m41 result 21: invokespecial #63; //Method java/lang/StringBuilder."<init>":(Ljava/lang/String;)V 24: aload_1 25: aload_2 26: if_acmpne 33 29: iconst_1 30: goto 34 33: iconst_0 34: invokevirtual #66; //Method java/lang/StringBuilder.append:(Z)Ljava/lang/StringBuilder; 37: invokevirtual #70; //Method java/lang/StringBuilder.toString:()Ljava/lang/String; 40: invokevirtual #74; //Method java/io/PrintStream.println:(Ljava/lang/String;)V 43: returnpublic void m42(); Code: 0: sipush 128 3: invokestatic #49; //Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer; 6: astore_1 7: sipush 128 10: invokestatic #49; //Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer; 13: astore_2 14: getstatic #53; //Field java/lang/System.out:Ljava/io/PrintStream; 17: new #59; //class java/lang/StringBuilder 20: dup 21: ldc #94; //String m42 result 23: invokespecial #63; //Method java/lang/StringBuilder."<init>":(Ljava/lang/String;)V 26: aload_1 27: aload_2 28: if_acmpne 35 31: iconst_1 32: goto 36 35: iconst_0 36: invokevirtual #66; //Method java/lang/StringBuilder.append:(Z)Ljava/lang/StringBuilder; 39: invokevirtual #70; //Method java/lang/StringBuilder.toString:()Ljava/lang/String; 42: invokevirtual #74; //Method java/io/PrintStream.println:(Ljava/lang/String;)V 45: return} 分析

javap是Java自带的一个工具,可以反编译,也可以查看Java编译器生成的字节码(上面代码只使用了javap -c DataType),是分析代码的一个好工具,具体怎么使用请Google一下

先看一下m4,为什么运行结果中出现了“true”呢,true说明a、b是同一个对象。

但a对象是调用Integer.valueOf()生成的,b是通过自动装箱生成的对象,为什么会是同一个对象呢?再看一下字节码吧,毕竟Java程序是依靠虚拟机运行字节码实现的。

m41这个方法只适用了一次valueOf(),但字节码中出现了两次,说明自动装箱时也调用了valueOf()。

下面是valueOf()具体实现

/** * Returns a <tt>Integer</tt> instance representing the specified * <tt>int</tt> value. * If a new <tt>Integer</tt> instance is not required, this method * should generally be used in preference to the constructor * {@link #Integer(int)}, as this method is likely to yield * significantly better space and time performance by caching * frequently requested values. * * @param i an <code>int</code> value. * @return a <tt>Integer</tt> instance representing <tt>i</tt>. * @since 1.5 */ public static Integer valueOf( int i) { final int offset = 128 ; if (i >= - 128 && i <= 127 ) { // must cache return IntegerCache.cache[i + offset]; } return new Integer(i); }

在【-128,127】之间的数字,valueOf返回的是缓存中的对象,所以两次调用返回的是同一个对象。

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